There are no separate accusative or dative cases in English. There is only a single objective case (which is sometimes called “accusative"), and that only for a few pronouns. The dative, and most of the accusative, case disappeared in the early Middle English period, leaving only the nominative (or “common") case and the genitive (or “possessive") case for nouns. Some pronouns retained an objective case. I think your grammar teacher was trying to graft Latin rules onto English.

The which/that distinction is maintained in edited prose, but ignored by everyone else.

It isn’t maintained in edited prose, according to MWDEU. If we’re talking about the same thing - I’m talking about how edited prose allows both which and that in restrictive clauses but only which with non-restrictive clauses.

We are talking about the same thing. There is a distinction between which and that, as you describe, in edited prose. In casual writing it is ignored and that is used with restrictive clauses.

That Old English example is dative plural, not genitive. (The -um ending is a dead giveaway for dative plural.)

Matthew, the blessed Evangelist, wrote in this evangelical lecture, that “Jesus came down from a mountain, and a great multitude followed him. Behold, there came a leprous man, and fell down before Jesus, thus saying, Lord, if thou wilt, thou canst cleanse me. Jesus stretched forth his hand, and touched him, and said, I will; and be thou cleansed. Then immediately was his leprosy all cleansed, and he was healed. Then said Jesus to him, Take care that thou say it to no man; but go to God’s temple, and show thyself to the priest, and offer thy gift, as Moses commanded for a witness to them.”

This makes it look as if “nanum men” means “to no man,” which would be singular. I haven’t taken the time to search all the way through Boethius. I looked at Chapter 11 section 1 cursorily but didn’t find the passage. It may be right there but I didn’t recognize it.

It strikes me that if “nan” in Anglo-Saxon is an adjective, as it appears to be in the examples above, it is analogous to the German “kein” as in “kein Mann” or “no man.” Is this the same meaning as “none”? Maybe so. In that case “none” does not derive from “no one” at all but was instead an adjective that could take the singular or plural in any declension, i.e. nominative, dative, accusative, genitive.

The confusion, then, might be that in Chaucer’s time “no one” (pronounced “no-ohn") collapsed into “none” and then got conflated into the pre-existing “none.” Just a speculation.

Nan can be none or not any. It is a blend of ne (not) + an (one). You’re right this isn’t the greatest example for comparison with the modern none because that word is no longer used adjectively—although it was well into the modern period, and adjectival use survives in the phrase none other.

A better example might be the from the Old English translation of the Rule of St. Benedict:

This makes it look as if “nanum men” means “to no man,” which would be singular.

“to no men”. It’s dative plural.

oððe hie næfre to nanum men ne becumaþ. oððe hi þær næfre fæstlice ne þurhƿuniaþ sƿelca sƿelce hi ær to coman.
“for either they never come to no men, or they never constantly remain such as they first came.”

Here’s an example where “none” is the subject of the plural verb “miȝten” (I think)

c1300 South English Legendary: Thomas Becket, Archbishop of Canterbury (LdMisc 108) 1182: His limes al-so he bi-heold, hou faire heo weren and freo, Þe hondene faire and longe fingres, fairore ne miȝten none beo.
“He also beheld his limbs, how fair and free they were, the fair hands and long fingers, none could be fairer.”

However, this would leave “for whom the bell tolls” as incorrect. I remember our teacher being a stickler on this. He always corrected the use of ‘for’, saying “who is this for?” not ‘whom is this for”?

Unfortunately, if you substitute he and him etc. for who and whom to resolve the issue, it doesn’t help since ‘him’ takes on both Accusative and Dative cases.

If you’re going to rearrange the phrase to figure it out, it would be ‘The bell tolls for him” or “the bell tolls for he.”

The thing you have to remember about grammar is that the “rules” have to conform to how people speak, not the other way around. The living language is the real grammar, not “rules.” And since language is always in a state of flux, any “rule” that can’t keep up falls by the wayside. Grammar is the servant, not the master.

I was taught (back in the fifties) that who and whom, he and him etc., were governed by the preposition in the sentence. Some prepositions take on the Accusative case, while others the Dative.... Dative is always whom, etc., while Accusative is who.

I think you may have had one of those teachers who take advantage of their position to teach their personal crotchets as standard rules of grammar.

Matthew, the blessed Evangelist, wrote in this evangelical lecture, that “Jesus came down from a mountain, and a great multitude followed him. Behold, there came a leprous man, and fell down before Jesus, thus saying, Lord, if thou wilt, thou canst cleanse me. Jesus stretched forth his hand, and touched him, and said, I will; and be thou cleansed. Then immediately was his leprosy all cleansed, and he was healed. Then said Jesus to him, Take care that thou say it to no man; but go to God’s temple, and show thyself to the priest, and offer thy gift, as Moses commanded for a witness to them.”

This makes it look as if “nanum men” means “to no man,” which would be singular. I haven’t taken the time to search all the way through Boethius. I looked at Chapter 11 section 1 cursorily but didn’t find the passage. It may be right there but I didn’t recognize it.

Nanum menn is plural, but an idiomatic translation into modern English would usually change it to present tense, because that’s the way modern English rolls.

If you could provide a citation, a website, and a credible translation it would help.

As the citation states, it’s from the beginning of chapter 11. The translation is: (either they never come to anyone (lit. to no men) or they steadfastly remain such as they were when they came.)